1. Field of the Invention
The present invention relates to ink jet printing apparatus, e.g. of the continuous type, which has lower print head structure, and more specifically to a structural and functional system that provides an improved storage mode for such apparatus.
2. Description of the Prior Art
The term "continuous" has been used in the field of ink jet printer apparatus to characterize the types of ink jet printers that utilize continuous streams of ink droplets, e.g. in distinction to the "drop on demand" types. Continuous ink jet printers can be of the binary type (having "catch" and "print" trajectories for droplets of the continuous streams) and of the multi-deflection type (having a plurality of print trajectories for droplets of the continuous streams). Binary type apparatus most often employs a plurality of droplet streams while multi-deflection apparatus most often employs a single droplet stream.
In general, continuous ink jet printing apparatus have an ink cavity to which ink is supplied under pressure so as to issue in a stream(s) from an orifice plate in liquid communication with the cavity. Periodic perturbations are imposed on the liquid stream(s), e.g. vibrations by an electromechanical transducer, to cause the stream(s) to break up into uniformly sized and shaped droplets. A charge plate is located proximate the stream(s) break-off point to impart electrical charge in accord with a print information signal and charged droplets are deflected from their nominal trajectory. In one common binary printing apparatus, charged droplets are deflected into a catcher assembly and non-charged droplets proceed along their nominal trajectory to the print medium.
The components described above (particularly the orifice plate and charge plate) should be precisely sized and positioned to achieve accurate placement of droplets on the print medium or catcher face. However, even after such careful manufacture, significant problems often are presented when the apparatus is shut-down for extended periods (e.g. overnight). That is, ink residue which remains from previous usage, will often dry on the orifice and charge plate structure during such shut-down periods. If the residue is in the orifice plate it can cause crooked jets. If residue is on the charge plate it can cause shorting or improper charging of droplets. Excessive residue on the other lower print head structure (e.g. the catcher) can cause disturbance in the droplet flight.
One approach for obviating the residue problem is for the operator to physically clean away the residue; however, this is not desirable for an office-environment printer. Also, operator cleaning often requires moving the charge plate, which is undesirable from the viewpoint of maintaining precise alignment.
Prior art solutions attempting to avoid operator cleaning have involved (i) providing a nearly instantaneous negative pressure at the shut-down of ink flow to avoid forming the residue on the charge plate and lower print head structure; (ii) purging the ink cavity and orifice plate with cleaning solution and/or air during a start-up or shut-down cycle; and (iii) providing a rapid pressure pulse in the image bar at start-up to force an initially straight start of the ink jets. These solutions are all helpful in avoiding ink residue problems without operator cleaning, but they are not without related difficulties and disadvantages. For example, introducing air or cleaning solution into the ink system adds considerable complexity to the apparatus and creates an additional operative cycle at shut-down and/or start-up. The "water-hammer" approach for achieving instantaneous start-up of the jets requires an extremely fast-actuation solenoid valve and rigid conduits, and it is sometimes unreliable in configurations where jet-to-electrode spacings are small. The instant shut-down procedure adds complexity to the fluid handling system and also can be unreliable.